For summarizing a data series, use the summary command, or its cousin fivenum. Fivenum gives the Tukey five number summary (minimum, lower-hinge, median, upper-hinge, maximum). Hinges are the medians of the left and right halves of the data, which is only slightly different than quartiles.

The two most common measures of central tendency are mean and median. Variance and standard deviation measure how much variation there is from the mean. They are measures of dispersion or spread.
The standard deviation, the square root of the variance, has the same units as the original data.

Other measures of variability or dispersion are quantiles (quantile) and inter-quartile range (IQR).

Histograms are a graphical way to look at how data points are distributed over a range. To construct a histogram, we first divide the data into bins. Then, for each bin, we draw a rectangle whose area is proportional to the frequency of data that falls into that bin. Drawing histograms in R is done with the hist command.